System and Method for Monitoring Oxygen Saturation

ABSTRACT

A method and system for monitoring oxygen saturation of a patient are provided. An example system includes a wearable device having a first optical sensor to measure a first red wavelength photoplethysmography (PPG) signal and a first infrared wavelength PPG signal and a second optical sensor to measure a second red wavelength PPG signal and a second infrared wavelength PPG signal. The system further includes a processor configured to determine that conditions for calibration of the first optical sensor are satisfied, determine a first ratio for obtaining the oxygen saturation, a first parameter for modifying the first red wavelength PPG signal, a second parameter for modifying the first infrared wavelength PPG signal, and a second ratio for obtaining the oxygen saturation. The processor is further configured to determine a value of the oxygen saturation and provide a message regarding a health status of the patient.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation of U.S. patent applicationSer. No. 16/132,224, titled “System and Method for MonitoringRespiratory Rate and Oxygen Saturation,” filed on Sep. 14, 2018, whichis a Continuation-in-Part of U.S. patent application Ser. No.14/738,666, titled “Monitoring Health Status of People Suffering fromChronic Diseases,” filed on Jun. 12, 2015, a Continuation-in-Part ofU.S. patent application Ser. No. 14/738,636, titled “Wearable DeviceElectrocardiogram,” filed on Jun. 12, 2015, a Continuation-in-Part ofU.S. patent application Ser. No. 14/738,711, titled “Pulse Oximetry,”filed on Jun. 12, 2015, and a Continuation-in-Part of U.S. patentapplication Ser. No. 14/983,118, titled “Using Invariant Factors forPulse Oximetry,” filed on Dec. 29, 2015. U.S. patent application Ser.No. 14/983,118 is a Continuation-in-Part of U.S. patent application Ser.No. 14/738,666, titled “Monitoring Health Status of People Sufferingfrom Chronic Diseases,” filed on Jun. 12, 2015, a Continuation-in-Partof U.S. patent application Ser. No. 14/738,636, titled “Wearable DeviceElectrocardiogram,” filed on Jun. 12, 2015, and a Continuation-in-Partof U.S. patent application Ser. No. 14/738,711, titled “Pulse Oximetry,”filed on Jun. 12, 2015. All of the disclosures of the aforementionedapplications are incorporated herein by reference for all purposes,including all references cited therein.

FIELD

The present application relates to systems and methods for monitoringmedical parameters of people, and more specifically to systems andmethods for monitoring respiratory rate and oxygen saturation.

BACKGROUND

It should not be assumed that any of the approaches described in thissection qualify as prior art merely by virtue of their inclusion in thissection.

Monitoring chronic diseases, which includes measuring medicalparameters, is central to providing appropriate and timely treatment topatients suffering from such chronic diseases as chronic heart failure,cardiac arrhythmia, chronic obstructive pulmonary disease, asthma, anddiabetes. Traditionally, monitoring is carried out and measurements aretaken while a patient is hospitalized or in other clinical settings.Appropriate treatment regimens can be based on these measurements, andthus it is highly beneficial to monitor medical parameters of thepatient after the patient is released from the hospital. Therefore, thepatient can be asked to visit the hospital or clinic periodically formonitoring and adjustment of treatment, if necessary.

However, most often, no measurements are carried out between visits,usually due to the need for trained examiners and medical devices. Thisis unfortunate, because, between visits, the chronic disease from whichthe patient suffers can worsen and result in emergency treatment andhospitalization. Furthermore, after receiving repeated courses ofemergency hospital treatment, the patient's health condition may degradeand never return to the pre-hospitalization level. Therefore, atechnology that allows for at-home measurements of medical parameterscan be essential to managing chronic diseases or even saving a patient'slife. Early warnings of worsening conditions associated with chronicdiseases may prevent unnecessary hospitalizations by providing apreventive treatment and, as a result, reduce financial and human costsof the hospitalization and treatment.

SUMMARY

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used as an aid in determining the scope of the claimed subjectmatter.

According to one embodiment of the present disclosure, a system formonitoring medical parameters of a patient is provided. The system mayinclude a wearable device configured to be disposed around a wrist ofthe patient. The wearable device may include a gyroscope configured toprovide a gyroscope signal indicative of a motion of the patient. Thesystem may further include a processor communicatively coupled to thegyroscope. The processor can be configured to perform a spectralanalysis of the gyroscope signal to obtain a spectrum in apre-determined range. The pre-determined range can cover a normalrespiratory rate range. The processor can be configured to determine aposition of a peak in the spectrum to obtain a value for the respiratoryrate. The processor can be configured to provide, based on the value ofthe respiratory rate, a message regarding a health status of thepatient.

Based on the determination that the value of the respiratory rate isoutside the normal respiratory range, the processor can provide an alertmessage regarding the health status of the patient. The spectralanalysis can be performed by a method of averaged periodograms. A normalrespiratory rate range may include 6 to 18 breaths per minute.

The processor can be configured to determine the strongest amplitudepeak in the spectrum to obtain a value for the respiratory rate. Theprocessor can determine that the spectrum includes one or more furtherpeaks with descending amplitudes. The further peaks correspond tofrequencies n*ω, wherein ω is a frequency corresponding to the strongestamplitude peak, and n is a natural number. Based on the determination,the processor may assign ω to the value of the respiratory rate.

The wearable device may further include a first optical sensorconfigured to measure, at a palmar surface of the wrist, a first redwavelength photoplethysmography (PPG) signal and a first infraredwavelength PPG signal. The wearable device may further include a secondoptical sensor configured to measure, at a dorsal surface of the wrist,a second red wavelength PPG signal and a second infrared wavelength PPGsignal. The first optical sensor and the second optical sensor can becommunicatively coupled to the processor.

The processor can be configured to determine, based on the first redwavelength PPG signal and the first infrared wavelength PPG signal, afirst ratio for obtaining an oxygen saturation. The processor can befurther configured to determine, based on the second red wavelength PPGsignal and the second infrared wavelength PPG signal, a second ratio forobtaining the oxygen saturation. The processor can be further configuredto determine, based on the first ratio and the second ratio, a thirdratio to be used for obtaining an oxygen saturation. The processor canbe further configured to determine, based on the third ratio, a value ofthe oxygen saturation. The processor can be further configured toprovide, based on the value of the oxygen saturation, a messageregarding a health status of the patient.

The third ratio can be determined by formula R=α R_(a)+(1−a)R_(b),wherein the a is a weight between 0 and 1, the R_(a) is the first ratio,and the R_(b) is the second ratio. The weight α can be a function of aperfusion index. The perfusion index can be determined based on thesecond red wavelength PPG signal or the second infrared wavelength PPGsignal. The weight α increases when the perfusion index increases. Ashape of the function is pre-determined during a calibration process.

The first optical sensor can be configured to measure the first redwavelength PPG signal and the first infrared wavelength PPG signalsubstantially near a radial artery of the wrist.

According to another example embodiment of the present disclosure, amethod for monitoring medical parameters of a patient is provided. Themethod may include providing, by a gyroscope integrated into a wearabledevice configured to be worn around a wrist of a patient, a gyroscopesignal indicative of a motion of the patient. The method may furtherinclude performing, by the processor communicatively coupled with thegyroscope, a spectral analysis of the gyroscope signal to obtain aspectrum in a pre-determined range. The pre-determined range may cover anormal respiratory rate range. The method may further includedetermining, by the processor, a position of a peak in the spectrum toobtain a value for a respiratory rate. The method may further includeproviding, by the processor and based on the value of the respiratoryrate, a message regarding a health status of the patient.

The method can include measuring at a palmar surface of the wrist of thepatient, by a first optical sensor of the wearable device, a first redwavelength PPG signal and a first infrared wavelength PPG signal. Themethod may include measuring at a dorsal surface of the wrist, by asecond optical sensor of the wearable device, a second red wavelengthPPG signal and a second infrared wavelength PPG signal. The firstoptical sensor and the second optical sensor can be communicativelyconnected to the processor.

The method may further include determining, by a processor and based onthe first red wavelength PPG signal and the first infrared wavelengthPPG signal, a first ratio for obtaining an oxygen saturation.

The method may include determining, by the processor and based on thesecond red wavelength PPG signal and the second infrared wavelength PPGsignal, a second ratio to be used for obtaining the oxygen saturation.The method may further include determining, by the processor and basedon the first ratio and the second ratio, a third ratio for obtaining anoxygen saturation. The method may further include determining, by theprocessor and based on the third ratio, a value of the oxygensaturation. The method may further include providing, by the processorbased on the value of the oxygen saturation, a message regarding ahealth status of the patient.

According to another example embodiment of the present disclosure, thesteps of the method for monitoring medical parameters of a patient canbe stored on a non-transitory machine-readable medium comprisinginstructions, which when implemented by one or more processors performthe recited steps.

Other example embodiments of the disclosure and aspects will becomeapparent from the following description taken in conjunction with thefollowing drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments are illustrated by way of example and not limitation in thefigures of the accompanying drawings, in which like references indicatesimilar elements and in which:

FIG. 1 is a block diagram showing an example system for monitoringmedical parameters of a patient.

FIG. 2 is a block diagram showing components of an example device formonitoring medical parameters of a patient.

FIG. 3 is a block diagram illustrating example sensors, example medicalparameters, and example chronic diseases.

FIGS. 4A and 4B are schematic diagrams illustrating an example devicefor monitoring medical parameters of a patient.

FIG. 4C is a schematic diagram illustrating an example optical sensor.

FIG. 5 is a block diagram showing an example system for monitoringmedical parameters of a patient.

FIG. 6A is a plot of an example PPG signal.

FIG. 6B shows an example function 600 for determining the weight α basedon the value of a perfusion index.

FIG. 7 shows plots of a gyroscope signal, spectrum of the gyroscopesignal, reference signal, and spectrum of the reference signal.

FIG. 8 shows plots of another gyroscope signal, spectrum of anothergyroscope signal, another reference gyroscope signal, and spectrum ofanother reference gyroscope signal.

FIG. 9 is a flow chart showing steps of an example method for monitoringa respiratory rate.

FIG. 10 is a flow chart diagram illustrating an example method formonitoring respiratory rate of a patient.

DETAILED DESCRIPTION

The following detailed description includes references to theaccompanying drawings, which form a part of the detailed description.The drawings show illustrations in accordance with exemplaryembodiments. These exemplary embodiments, which are also referred toherein as “examples,” are described in enough detail to enable thoseskilled in the art to practice the present subject matter. Theembodiments can be combined, other embodiments can be utilized, orstructural, logical and electrical changes can be made without departingfrom the scope of what is claimed. The following detailed descriptionis, therefore, not to be taken in a limiting sense, and the scope isdefined by the appended claims and their equivalents.

The present disclosure provides systems and methods for monitoringmedical parameters of people suffering from chronic diseases.Embodiments of the present disclosure can allow measuring medicalparameters of a patient in a non-intrusive manner while, for example,the patient is at home, at work, outdoors, traveling, and at otherstationary or mobile environments. Some example embodiments can providefor a wearable device (e.g., a wristband, a watch, or a bracelet) thatincludes sensors configured to measure medical parameters such as, forexample, oxygen saturation, respiratory rate, and the like. Themeasurements can be taken during daytime and nighttime for days, weeks,months, and years. The medical parameters can be analyzed to determinetrends in the medical parameters and to determine whether the severityof the patient's chronic disease (e.g., a heart disease, diabetes, lungdisease, and so on) worsens or improves. Embodiments of the presenttechnology may facilitate a rapid reaction to provide an appropriate andtimely treatment for the patient. The early treatment may allow takingtimely preventive measures to avoid worsening of the patient's conditionto the point of requiring an emergency hospitalization and associatedexpensive medical treatment.

According to various example embodiments, a method for monitoringmedical parameters of a patient can include measuring at a palmarsurface of a wrist of the patient, by a first optical sensor of awearable device configured to be disposed around the wrist of thepatient, a first red wavelength PPG signal and a first infraredwavelength PPG signal. The method may include measuring from a dorsalsurface of the wrist, by a second optical sensor, a second redwavelength PPG signal and a second infrared wavelength PPG signal. Themethod may also include determining, by a processor communicativelyconnected to the first optical sensor and the second optical sensor andbased on the first red wavelength PPG signal and the first infraredwavelength PPG signal, a first ratio for obtaining an oxygen saturation.

The method may include determining, by the processor and based on thesecond red wavelength PPG signal and the second infrared wavelength PPGsignal, a second ratio to be used to obtain the oxygen saturation. Themethod may further include determining, by the processor and based onthe first ratio and the second ratio, a third ratio to be used to obtainan oxygen saturation. The method may further include determining, by theprocessor and based on the third ratio, a value of the oxygensaturation. The method may further include providing, by the processorbased on the value of the oxygen saturation, a message regarding ahealth status of the patient.

The method may further include providing, by a gyroscope communicativelycoupled with the processor and disposed in the wearable device, agyroscope signal indicative of motion of the patient. The method mayfurther include performing, by the processor, a spectral analysis of thegyroscope signal to obtain a spectrum in a pre-determined range. Thepre-determined range may cover a normal respiratory rate range. Themethod may further include determining, by the processor, a position ofa peak in the spectrum to obtain a value for a respiratory rate. Themethod may further include determining, by the processor, that the valueof the respiratory rate is outside of the normal respiratory rate range.If the value of the respiratory rate is outside of the normalrespiratory rate range, the method may further include providing, by theprocessor, an alert message regarding the health status of the patient.

Referring now to FIG. 1, an example system 100 for monitoring apatient's health status is shown. The system 100 includes at least awearable device 110. The wearable device may include sensors 120. Insome embodiments, the wearable device 110 can be worn by a patient 130,for example on a wrist, for an extended period of time. The wearabledevice 110 can be carried out as a watch, a bracelet, a wristband, andthe like.

The wearable device 110 can be configured to constantly collect, viasensors 120, sensor data from a patient 130. In some embodiments, basedon the sensor data, a processor of the wearable device 110 can beconfigured to obtain, based on the sensor data, medical parametersassociated with the patient 130. The medical parameters can be analyzedto obtain changes (trends) in medical parameters over time. Based on thechanges, one or more conclusions regarding severity of one or morechronic diseases can be obtained. The processor of the wearable device110 can be configured to send, via a communication module of thewearable device 110, messages regarding a current health status to acomputing device of the patient, a relative, a caretaker of the patient,or a doctor treating the patient. The message to the patient may includean advice to see a doctor and/or take medicine. The processor of thewearable device 110 can display the message on a graphical displaysystem of the wearable device 110.

In some embodiments, the system 100 includes a mobile device 140. Themobile device 140 can be communicatively coupled to the wearable device110. In various embodiments, the mobile device 140 is operable tocommunicate with the wearable device 110 via a wireless connection suchas, for example, Wi-Fi, Bluetooth, Infrared (IR), and the like. Themobile device 140 can include a mobile phone, a smart phone, a phablet,a tablet computer, a notebook, and so forth. The mobile device 140 canbe configured to receive the sensor data and medical parameters from themobile device 140. In certain embodiments, a processor of the mobiledevice can be configured to perform analysis of the received sensor datato determine medical parameters. The mobile device 140 can be furtherconfigured to provide, via a graphic display system of the mobile device140, a report regarding current health status. In various embodiments,the mobile device 140 runs one or more applications that provide, viathe graphical display system of the mobile device 140, charts andgraphics concerning medical parameters of the patient.

In some embodiments, the mobile device 140 can be configured todetermine the severity of a health status resulting from the chronicdisease from which the patient suffers. The mobile device can beconfigured to provide the patient with advice to see a medicalprofessional or to take medicine. An alert message regarding healthstatus of the patient can be sent, by the mobile device 140, to acomputing device of a doctor, relative, or caretaker of the patient.

In further embodiments, the system 100 may further include a cloud-basedcomputing resource (also referred to as a computing cloud 150). In someembodiments, the cloud-based computing resource includes one or moreserver farms/clusters comprising a collection of computer servers and isco-located with network switches and/or routers. In certain embodiments,the mobile device 140 and/or wearable device 110 can be communicativelycoupled to the computing cloud 150. The mobile device 140 and/orwearable device 110 can be operable to send the sensor data and medicalparameters to the computing cloud 150 for further analysis. Thecomputing cloud 150 can be configured to store historical dataconcerning patient health status including sensor data and medicalparameters collected over days, weeks, months, and years. The computingcloud 150 can be configured to run one or more applications to providereports regarding health status of the patient. A doctor 170 treatingthe patient may access the reports, for example via a computing device160, using the Internet or a secure network. In some embodiments, theresults of the analysis of the medical parameters can be sent back tothe mobile device 140 and/or wearable device 110.

The severity of the health status resulting from a chronic disease canbe estimated by computing a deviation or divergence from normal medicalparameters of one or more medical parameters being measured at themoment. The normal medical parameters can be specific to the patient andcan be derived based on historical data concerning the patient's healthstatus recorded over an extended time period. If the deviation in themedical parameters becomes sufficiently large, the patient can beadvised, via a message to the mobile device 140, to take medicine orcontact a doctor. In some situations, when the deviation becomessubstantial, an alert message can be sent by the mobile device 140and/or the wearable device 110 to a computing device of a relative, adoctor, or a caretaker of the patient.

It may be desirable for the patient to be assured that the currentmedical parameters are within an acceptable deviation of the normalmedical parameters. For example, when the current medical parameters arenormal, the wearable device 110 and/or mobile device 140 can be operableto periodically alert the patient using a pleasant sound. The signal canbe provided, for example, every 30 minutes, once every hour, and thelike. In certain embodiments, when the medical parameters are withinnormal values, the mobile device 140 may provide a text message assuringthe patient of normal conditions. A haptic feedback component can beused to alert the patient to a health condition, to warn the patientabout a specific event concerning treatment of a chronic disease, toremind the patient to take a medicine, if the patient has failed to takethe medicine within a pre-determined period of time, and so forth. Thewearable device 110 may include a haptic feedback functionality forproviding the patient with haptic feedback, for example, a tap-indevice, to apply a force or vibration to skin of the patient. In furtherembodiments, the haptic alert can be provided by the mobile device 140.The mobile device 140 can vibrate when the mobile device is in a pocketof the patient or when the mobile device 140 is located on a surface(e.g., a table).

FIG. 2 is a block diagram illustrating components of wearable device110, according to an example embodiment. The example wearable device 110may include sensors 120, a communication unit 210, a processor 220,memory 230, and a battery 240.

The communication unit 210 can be configured to communicate with anetwork such as the Internet, a Wide Area Network (WAN), a Local AreaNetwork (LAN), a cellular network, and so forth, to send a data stream,for example sensor data, medical parameters, and messages concerning thehealth condition of a patient.

The processor 220 can include hardware and/or software, which isoperable to execute computer programs stored in memory 230. Theprocessor 220 can use floating point operations, complex operations, andother operations, including processing and analyzing sensor data.

In some embodiments, the battery 240 is operable to provide electricalpower for operation of other components of the wearable device 110. Insome embodiments, the battery 240 is a rechargeable battery. In certainembodiments, the battery 240 is recharged using inductive chargingtechnology.

The wearable device 110 may include additional or different componentsto provide a particular operation or functionality. Similarly, in otherembodiments, the wearable device 110 includes fewer components thatperform similar or equivalent functions to those depicted in FIG. 2. Forexample, the wearable device 110 may further include a graphical displaysystem to provide an alert message and display current values of themedical parameters to the patient. The wearable device 110 may alsoinclude a haptic device to alert the patient with vibrations. Thewearable device 110 may also include an audio signaling device, forexample, a beeper or a speaker, to provide sound alerts to the patient.

FIG. 3 is a block diagram showing a list of example sensors 120, a listof example medical parameters 310, and a list of example chronicdiseases 320. In various embodiments, the sensors 120 may includeoptical sensors 222 a and 222 b, electrical sensors 224, and motionsensors 226. The medical parameters 310, determined based on the sensordata, include, but are not limited to, SpO₂ oxygen saturation, tissueoxygen saturation, cardiac output, vascular resistance, pulse rate,blood pressure, respiration, electrocardiogram (ECG) data, respiratoryrate, and motion data. The chronic diseases 320, the progression ofwhich can be tracked based on changes of the medical parameters, includebut are not limited to congestive heart failure (CHF), hypertension,arrhythmia, asthma, chronic obstructive pulmonary disease (COPD),hypoglycemia, sleep apnea, and Parkinson's disease.

The optical sensors 222 a and 222 b can be operable to measure medicalparameters associated with blood flow in blood vessels using changingabsorbance of light at different wavelengths in blood and skin. The datafrom optical sensors 222 a and 222 b can be used to determine multiplemedical parameters, including but not limited to: SpO₂ oxygensaturation, cardiac output, vascular resistance, pulse rate, andrespiratory rate. Based on the measurements obtained from opticalsensors 222 a and 222 b, abnormal cardiac rhythms (for example, atrialfibrillation, rapid rhythms, and slow rhythms) can be detected.

In some embodiments, respiratory rate can be derived from a sinusarrhythmia waveform. The sinus arrhythmia waveform can be obtained basedon intervals between subsequent heart beats (RR intervals) measured bythe optical sensors 222 a and 222 b using the fact that the rhythm ofthe heart beats is modulated by human breathing.

The electrical sensors 224 can be operable to obtain ECG activity dataof the patient. The ECG activity data includes a characteristicelectrically-derived waveform of a heart activity. The ECG data caninclude a number of components, whose characteristics (timing,amplitude, width, and so forth), alone or in combination, can provide apicture of cardiac and overall health. The ECG data is typically derivedby measurements from one or more sets of leads (groups of electrodescomprising grounded circuits), such that the exact characteristics ofthe resulting waveforms is a function of the electrical and spatialvectors of the electrode positions relative to the heart. While thedetails of interpretation of the ECG data are too involved to describesuccinctly in this disclosure, consideration of each of the componentparameters can indicate health status, physical or psychological stress,or trends of disease conditions. Various cardiovascular parameters canbe extracted from the ECG alone (such as a heart rate for example), orin combination with other physiological measurements.

According to example embodiments of present disclosure, ECG of thepatient can be measured via the electrical sensors 224. Sincemeasurements are taken from a wrist of the patient, electrodes of theelectrical sensors 224 should be located very close to each other on awearable device. Therefore, the ECG data may contain noise. Averaging ofseveral subsequent intervals of the ECG data between heart beats can beused to cancel out noise in ECG data. To determine intervals between twosubsequent heart beats, the PPG signals as measured by optical sensors222 a and 222 b can be used as a reference. In some embodiments, anarrhythmia analysis can be carried out using the ECG data and dataconcerning cardiac output and pulse rate.

In some embodiments, motion sensors 226 include an accelerometer, agyroscope, and an Inertial Measurement Unit (IMU). The motion dataobtained via motion sensors 226 can provide parameters of body movementand tremor. The motion data can allow tracking the progression orremission of a motor disease, Parkinson's disease, and physicalcondition of the patient. In some embodiments, the motion data can beanalyzed to determine whether the patient is likely to fall. In someembodiments, the motion data can be analyzed in time domain andfrequency domain. By tracking amplitudes of movement of the patient itcan be determined if the patient's movements become slower (i.e., thepatient becomes sluggish) or the patient is not moving at all.Additionally, according to example embodiments of the presentdisclosure, the motion data can be analyzed to determine a respiratoryrate of the patient.

FIG. 4A and FIG. 4B are schematic diagrams illustrating an examplewearable device 110. In the examples of FIG. 4A and FIG. 4B, thewearable device 110 can be carried out in a shape of an open bangle. TheFIG. 4A shows a dorsal side of a patient's hand 410 and the wearabledevice 110 placed on the patient's wrist. FIG. 4B shows a palmar side ofthe patient's hand 410 and wearable device 110. The wearable device 110can be designed to apply pressure at an area of skin surface covering aradial artery 420. In comparison to wristbands and straps, an openbangle may be more comfortable to wear by a patient since no pressure isapplied to the middle area inside the wrist.

The wearable device 110 can include optical sensors 222 a and 222 blocated on an inner side of the wearable device 110. When the wearabledevice 110 is worn on the patient's hand, the inner side of the wearabledevice 110 can be in continuous contact with a surface of the skin ofthe patient's hand 410.

When the wearable device 110 is disposed around a wrist of patient'shand 410, the optical sensor 222 a is located as close as possible tocover the skin area covering an artery of the wrist, for example theradial artery 420. When the wearable device 110 is disposed around awrist of patient's hand 410, the optical sensor 222 b is located on thedorsal side of the wrist. The optical sensors 222 a and 222 b can beconfigured to be in a continuous contact with the skin of the patient130.

As shown in FIG. 4C, the optical sensors 222 a may include multiplelight sensors 440 (photodetectors), to measure the reflected light, andmultiple light transmitters 450 (for example, Light Emission Diodes(LEDs)). The number and location of the light sensors 440 and lighttransmitters 450 can be chosen such that in case of an accidentaldisplacement of the wearable device, at least one of the light sensorsis still located sufficiently close to the radial artery. In someembodiments, when measuring the light reflected from the skin and radialartery, a signal from those photoelectric cells that provides thestrongest or otherwise determined best output can be selected forfurther processing in order to obtain medical parameters using methodsof pulse (reflectance) oximetry. In certain embodiments, the wearabledevice 110 can be configured to apply a pre-determined amount ofpressure to the wrist each time the user wears the wearable device toallow the same conditions for the reflection of the light from the skin.The optical sensor 222 b may include elements analogous to the elementsof the optical sensor 222 a. The signals measured by optical sensors 222a and 222 b can be used to determine oxygen saturation, heart rate,cardiac output, and other parameters using pulse oximetry methods.

Oxygen saturation is the relative proportion (typically expressed aspercentage) of oxygen dissolved in blood, as bound to hemoglobin,relative to non-oxygen-bound hemoglobin. Oxygen saturation is importantin clinical monitoring of surgical anesthesia, and in monitoring andassessment of various clinical conditions such as COPD and asthma. Inhealthy individuals, oxygen saturation is over 95%. Direct measurementcan be made from arterial blood sample, but drawing blood is an invasiveprocedure, and, except for a few controlled environments (e.g., during asurgery) cannot be easily performed continuously. Pulse oximetry canyield a quantity called SpO2 (saturation of peripheral oxygen), anaccepted estimate of arterial oxygen saturation, derived from opticalcharacteristics of blood using transmission of light through a thin partof a human body (for example, a fingertip or an earlobe (in the mostcommon transmissive application mode)). Reflectance pulse oximetry canbe used to estimate SpO2 using other body sites. The reflectance pulseoximetry does not require a thin section of the person's body and istherefore suited to more universal application such as the feet,forehead, and chest, but it has some serious issues due to the lightreflected from non-pulsating tissues. When oxygen saturation cannot bemeasured directly from arterial blood, an indirect measurement can beperformed by tracking tissue oxygen saturation. The measurement ofoxygen saturation is commonly used to track progression of heart diseaseor lung disease. When the heart or lungs are not functioning properly,the saturation of oxygen drops in both arterial blood and tissue aroundthe artery. Therefore, tissue oxygen saturation can be measured bysensing the skin color near the radial artery. For example, if thewearable device 110 moves so that optical sensor 222 a is not coveringthe radial artery, measurements of tissue saturation near the radialartery can be used as a backup to provide values for oxygen saturation.

The optical sensor 222 a can be configured to measure an infraredwavelength PPG signal I_(a) ^(ir) and a red wavelength PPG signal I_(a)^(red) near a blood artery of the wrist of the patient (for example, theradial artery 420). The infrared wavelength PPG signal can be obtainedby emitting, by the light transmitters 450, an infrared wavelength lightand detecting, by the light sensor 440, intensity of infrared lightreflected from the skin and blood vessels of the patient. The redwavelength PPG signal can be obtained by emitting, by the lighttransmitters 450, a red wavelength light and detecting, by the lightsensor 440, intensity of red light reflected from the skin and bloodvessels of the patient. Similarly, the optical sensor 222 a can beconfigured to measure an infrared wavelength PPG signal I_(b) ^(ir) anda red wavelength PPG signal I_(b) ^(red) from the palmar side of thewrist of patient.

The wearable device 110 can include a gyroscope 430. The gyroscope 430can be located at any point within the wearable device 110. Thegyroscope may include a triple axis Micro Electro-Mechanical Systems(MEMS) gyroscope. The gyroscope 430 can measure rotation around threeaxes: x, y, and z.

In some embodiments, a gyroscope signal measured by the gyroscope 430can be analyzed to determine a respiratory rate of a patient. Therespiratory rate, which is a vital sign, is typically expressed as thenumber of breaths per minute. Typical adult resting respiratory rate isabout 16-20 breaths per minute. Extreme variations can result fromphysical or psychological stress. The respiratory rate is often affectedin chronic disease states, particularly in pulmonary and cardiovasculardisease. Extreme short-term changes may be associated with acute diseaseepisodes, particularly in chronically ill patients.

FIG. 5 is a block diagram showing components of a system 500 formonitoring health status of people suffering from chronic diseases,according to an example embodiment. The system 500 can include a sensordata acquisition module 510, an oxygen saturation estimation module 520,a respiratory rate estimation module 530, and an output module 540. Insome embodiments, the modules 510-540 are implemented as chipsetsincluded in the wearable device 110. In other embodiments, the modules520, 530, and 540 can be stored as instructions in memory of thewearable device 110, mobile device 140, or computing cloud 150, andexecuted by a processor.

In some embodiments, the sensor data acquisition module 510 isconfigured to receive and digitize the sensor data. The sensor dataacquisition module 510 can include one or more analog-to-digitalconverters to transform the electrical signals from sensors 120 todigital signals and provide the digital signal to modules 520-540 forfurther analysis.

In some embodiments, the data processing module is configured to analyzethe infrared wavelength PPG signals and the red wavelength PPG signalsrecorded by the optical sensor 222 a and 222 b to obtain oxygensaturation using the methods of pulse oximetry.

The methods for pulse oximetry are based on the fact that oxygenatedhemoglobin absorbs more infrared light while deoxygenated hemoglobinabsorbs more red light. Typically, the methods of the pulse oximetryinclude calculation of a ratio R defined as follows:

$\begin{matrix}{R = \frac{\log\left( \frac{I_{\max}^{red}}{I_{\min}^{red}} \right)}{\log\left( \frac{I_{\max}^{ir}}{I_{\min}^{ir}} \right)}} & (1)\end{matrix}$

wherein I_(max) ^(red) and I_(min) ^(red) are maximum and minimum of redwavelength PPG signal and I_(max) ^(ir) and I_(min) ^(ir) are maximumand minimum of infrared wavelength PPG signal. The ratio R can indicatea ratio between oxygenated hemoglobin and deoxygenated hemoglobin. Theratio R can be converted to a corresponding oxygen saturation (SpO₂)value via an empirically-derived look-up table.

In some embodiments, to take into account contributions due tointeraction of light with non-pulsatile tissue, the infrared wavelengthPPG signal can be shifted by a parameter L^(ir) and the red wavelengthPPG signal can be shifted by a parameter L^(red). The ratio R₁ can bethen determined as

$\begin{matrix}{{R_{1}\left( {L^{red},L^{ir}} \right)} = \frac{\log\left( \frac{I_{\max}^{red} - L^{red}}{I_{\max}^{red} - L^{red}} \right)}{\log\left( \frac{I_{\max}^{ir} - L^{ir}}{I_{\min}^{ir} - L^{ir}} \right)}} & (2)\end{matrix}$

The ratio R₁ can be further used to obtain the oxygen saturation via anempirically-derived look-up table or function. The parameters L^(red)and L^(ir) can be pre-determined in a calibration process. Thecalibration process may include optimization of L^(red) and L^(ir) tofulfill the equation R₁(L^(red), L^(ir))=R_(true), wherein the R_(true)is a true value for the ratio R found by a “gold standard” measurement.

Using the infrared wavelength PPG signal I_(a) ^(ir) and the redwavelength PPG signal I_(a) ^(red) recorded by the optical sensor 222 afrom palmar side of a wrist near a blood artery, the oxygen saturationestimation module 520 may be configured to calculate a ratio R_(a).Using the infrared wavelength PPG signal I_(b) ^(ir) and red wavelengthPPG signal I_(b) ^(red) recorded by the optical sensor 222 b from dorsalside of the wrist, the oxygen saturation estimation module 520 may beconfigured to calculate a ratio R_(b). The PPG signals I_(a) ^(ir),I_(a) ^(red), I_(b) ^(ir), and I_(b) ^(red) can be recordedsubstantially simultaneously.

Since the ratio R_(a) is determined from PPG signals recorded by opticalsensor 222 a from the palmar side of the wrist near a blood artery, thevalue of the ratio R_(a) may be sensitive to location of lighttransmitters 450 and light sensors 440 relative to the blood artery.Typically, error of value oxygen saturation (SpO₂) determined based onratio R_(a) decreases with increase of values for oxygen saturations.However, the optical sensor 222 a located at the radial artery on thepalmar side of the wrist may provide a PPG signals with aSignal-to-Noise Ratio (SNR) higher than the SNR of the PPG signal foroptical sensor 222 b located on the dorsal side of the wrist. Therefore,a location of an optical sensor near the radial artery is preferableover locations near fingertip or dorsal side of the wrist. Opticalsensors located at a fingertip or on the dorsal side of the wrist canmeasure PPG signals received from small blood vessels. The quality ofthe PPG measurements received from the small blood vessels may bedecreased due to several factors. One of the factors causing decrease inthe quality of the PPG measurements includes possible constriction ofsmall vessels due to low ambient temperatures. Another factor causingdecrease in the quality of the PPG measurements includes a darker colorof the skin covering the small blood vessels, which may lead to adecrease of the SNR in the PPG signals.

The ratio R_(b) is determined from PPG signals recorded at dorsal sideof the wrist, which does not include pulsatile blood vessels. Therefore,the ratio R_(b) can be assumed to be independent on location of thelight transmitters and light sensors of the optical sensor 222 brelative to the surface of the dorsal side of the wrist. When PPGsignals I_(b) ^(ir), and I_(b) ^(red) recorded by the optical sensor 222b are of a good quality, that is having a high SNR (as defined above),then the ratio R_(b) obtained using the PPG signals I_(b) ^(ir), andI_(b) ^(red) can be used to calibrate the PPG signals I_(a) ^(ir) andI_(a) ^(red) recorded by the optical sensor 222 a.

In some embodiments, the ratio R_(a) can be found by using the formula(2) and the ratio R_(b) can be found by using formula (1). The ratioR_(b) can be further used as R_(true) (the “gold standard” measurement)to optimize the parameters L^(red) and L^(ir) in formula (2). Thus, thePPG signals measured by the optical sensor 222 b on the dorsal side ofthe wrist can be used to calibrate the optical sensor 222 a on thepalmar side. The calibration process can be carried out each time thepatient wears the wearable device 110. The calibration process can bealso performed if a value of the oxygen saturation determined by usingthe PPG signals measured by the optical sensor 222 a is outside apre-determined range.

In some embodiments, the ratio R_(b) can be used to correct a value forratio R_(a). For example, the ratio R_(a) and ratio R_(b) can be fusedto obtain a ratio R. The ratio R can be further used to determine valueof the oxygen saturation via an empirically-derived look-up table.

In some embodiments, a ratio R can be determined as

R=αR _(a)+(1−α)R _(b)  (3)

wherein α is a weight, 0<α≤1. The weight α can be determined as afunction of SNR of PPG signal I_(b) ^(ir) or PPG signal I_(b) ^(red)measured at the palmar side of the wrist. Higher values of the weight αmay correspond to higher values of the SNR of PPG signals I_(b) ^(ir),or I_(b) ^(red) since a higher SNR signal is more reliable fordetermining ratio R. A weighted average of the ratio R_(a) and ratioR_(b) can provide a better estimate for the ratio R because measurementerrors of the optical sensor 222 a and 222 b are independent of eachother. At the same time, the ratio R_(a) can be given a larger weightbecause a measurement obtained on the palmar side of the wrist and nearthe radial artery may provide more information for core blood pressureand oxygen saturation than a measurement obtained from small vessels onthe dorsal side of the wrist.

In some other embodiments, weight α can be determined as a function of aperfusion index. The perfusion index is a ratio of the pulsatile bloodflow to the non-pulsatile static blood flow in patient's peripheraltissue. The perfusion index can be estimated based on a PPG signal I_(b)^(ir) or a PPG signal I_(b) ^(red) measured at the dorsal side of thewrist.

FIG. 6A shows a plot 600A of an example PPG signal 610 which representsan amount of light received by a photodetector of optical sensor 222 b.The perfusion index can be calculated as a ratio of difference 620 ofthe amount of light received by the photodetector betweenvasoconstriction and vascular dilation to the total amount 630 of lightreceived by the photodetector. Typically, the larger the perfusionindex, the more accurate the measurement of oxygen saturation.Therefore, the perfusion index can be used as an indicator of quality ofthe PPG signal.

If the perfusion index determined based on PPG signals PPG signal I_(b)^(ir) and PPG signal I_(b) ^(red) measured at the dorsal side of thewrist are higher than a pre-determined threshold, the ratio R_(b)obtained using the PPG signals I_(b) ^(ir) and I_(b) ^(red) can be usedto correct the ratio R_(a) using formula (3). Furthermore, if theperfusion index determined based on the PPG signal I_(b) ^(ir) and PPGsignal I_(b) ^(red) measured at the dorsal side of the wrist are higherthan a pre-determined threshold, the ratio R_(b) can be also used todetermine parameters L^(red) and L_(ir) in formula (2) for shifting PPGsignals I_(a) ^(red) and I_(a) ^(ir) and, thereby, calibrate the opticalsensor 222 a disposed at the palmar side of the wrist.

FIG. 6B shows an example function 600B for determining the weight αbased on the value of perfusion index. The value of the perfusion indexcan be found based on the PPG signal I_(b) ^(ir) or the PPG signal I_(b)^(red) measured at the dorsal side of wrist of a patient. The shape offunction 600 can be determined in a calibration process. The weight αincreases with an increase of the value for the perfusion index.

In some embodiments, the respiratory rate estimation module 530 isconfigured to analyze the gyroscope signal (rotation around three axesx, y, and z) measured by the gyroscope 430 at a wrist to obtainrespiratory rate of a patient. It was found in tests performed byinventor that the gyroscope signal measured at the wrist of the patientincludes data due to a cyclical motion of the chest of the patient.During the tests, a gyroscope signal measured by a gyroscope disposed ata wrist of a patient and a reference gyroscope signal measured by agyroscope disposed in a chest strap at the chest of the patient wererecorded simultaneously. A spectral analysis of the gyroscope signalmeasured at the wrist and the reference gyroscope signal measured at thechest were performed. A comparison of the spectra of the signals showsthat the gyroscope signal measured at the wrist contains a frequencycorresponding to the number of breaths of patient as determined based onthe reference gyroscope signal measured at the chest.

FIG. 7 shows plot 700 of example gyroscope signal 710 measured at awrist, spectrum 720 of the gyroscope signal 710, reference gyroscopesignal 730 measured at the chest, and spectrum 740 of the referencegyroscope signal 730. The spectrum 720 of the gyroscope signal 710includes a peak 750, which corresponds to a peak 760 in spectrum 740 ofthe reference gyroscope signal 730. The frequency at the peak 750corresponds to the respiratory rate.

FIG. 8 shows plot 800 of example gyroscope signal 810 measured at awrist, spectrum 820 of the gyroscope signal 810, reference gyroscopesignal 830 measured at the chest, and spectrum 840 of the referencegyroscope signal 830. The SNR of gyroscope signal 810 is higher than SNRof the gyroscope signal 710 shown in FIG. 7. The spectrum 820 of thegyroscope signal 810 includes a peak 850 which corresponds to peak 860in spectrum 840 of the reference gyroscope signal 830. The frequency atthe peak 850 corresponds to the respiratory rate.

The respiratory rate estimation module 530 may receive the gyroscopesignal measured by the gyroscope 430 at the wrist of the patient andperform the spectral analysis of the gyroscope signal to obtain aspectrum of the gyroscope signal in a pre-determined range. Thepre-determined range can be selected to cover a range of typical valuesfor the respiratory rate of a human (for example, 6-18 breaths perminute). For example, the pre-determined range may include a range of 0to 50 breaths per minute. In some embodiments, the spectral analysis mayinclude fast Fourier transform, averaged periodograms (Bartlett'smethod), Welch's method, least-squares spectral analysis, and so forth.

The respiratory rate estimation module 530 can be further configured todetermine a position of a peak in the spectrum that corresponds to thenumber of breaths of a patient per minute (the respiratory rate). Insome embodiments, the module 530 can be configured to select thestrongest peak in the pre-determined range. As seen in FIG. 7 and FIG.8, the spectrum of the gyroscope signal can be contaminated by a noisecaused by movement of the patient. Typically, the noise is of a lowfrequency. Therefore, in the spectrum, the frequencies caused by thenoise may interfere with the frequency corresponding to the respiratoryrate.

The signal s(t) representing motion of breathing can be described byformula:

s(t)=Σ_(n=1) ^(∞) A _(n) f(n*ω*t)  (4)

wherein f is a periodic function, ω is a fundamental frequency, andA_(n) are descending amplitudes. In a frequency domain, the signal s(t)is represented by peaks at frequencies n*ω, wherein amplitudes of thepeaks descend with the increase of n. This pattern can be searched in aspectrum of the gyroscope signal measured by the gyroscope at a wrist ofpatient to confirm that a correct peak was assigned to the respiratoryrate.

After determining the strongest peak, the respiratory rate estimationmodule 530 can confirm that the strongest peak in the spectrumrepresents breathing of the patient and not caused by movement noise ofthe patient. The respiratory rate estimation module 530 can beconfigured to determine that the spectrum includes one or more furtherpeaks with descending amplitudes, such as the further peakscorresponding to frequencies n*ω, wherein ω is a frequency correspondingto the strongest peak, and n is a natural number. Based on thedetermination, the respiratory rate estimation module 530 can assignvalue ω to the value of the respiratory rate.

In some embodiments, the output module 540 is configured to providereports and alert messages regarding a health status of the patient. Theoutput module 540 may be configured to display via a graphical displaysystem of the wearable device 110 or the mobile device 140. The outputmodule 540 may determine that the respiratory rate of the patient isnear or outside of a typical range (for example, 6-18 breaths perminute). The output module 540 may also determine that the value ofoxygen saturation of the patient is below a pre-determined threshold. Inresponse to the determination, the output module 540 may provide analert message to the patient via a graphical display system of thewearable device 110 or/and mobile device 140. The alert message mayinclude an advice to take medication or contact a doctor. The outputmodule may also send a warning message, via the communication unit ofthe wearable device 110 and/or mobile device 140, to a computing deviceof a relative, a doctor, or a caretaker of the patient.

FIG. 9 is a flow chart diagram showing example method 900 for monitoringoxygen saturation of a patient. The method 900 may be implemented insystem 100. In block 910, the method 900 may commence with measuring ata palmar surface of a wrist of the patient, by a first optical sensor, afirst red wavelength PPG signal and a first infrared wavelength PPGsignal. The first optical sensor can be embedded into a wearable devicedisposed around the wrist of the patient.

In block 920, the method 900 may proceed with measuring, at a dorsalsurface of the wrist, by a second optical sensor disposed in thewearable device, a second red wavelength PPG signal and a secondinfrared wavelength PPG signal. The second red wavelength PPG signal andthe second infrared wavelength PPG signal can be measured substantiallysimultaneously with the first red wavelength PPG signal and the firstinfrared wavelength PPG signal.

In block 930, the method 900 may continue with determining, by aprocessor communicatively connected to the first optical sensor and thesecond optical sensor and based on the first red wavelength PPG signaland the first infrared wavelength PPG signal, a first ratio forobtaining an oxygen saturation. In block 940, the method 900 maydetermine, by the processor and based on the second red wavelength PPGsignal and the second infrared wavelength PPG signal, a second ratio forobtaining the oxygen saturation.

In block 950, the method 900 may determine, by the processor and basedon the first ratio and the second ratio, a third ratio for obtaining anoxygen saturation. In block 960, the method 900 may determine, by theprocessor and based on the third ratio, a value of the oxygensaturation. In block 970, the method 900 may include providing, by theprocessor and based on the value of the oxygen saturation, a messageregarding a health status of the patient. The message may include awarning against upcoming episode or worsening of a chronic disease ofthe patient, for which the oxygen saturation is one of the indicators.Upon receiving such a warning, preventive measures can be taken toreduce the severity of upcoming or worsening of the chronic disease. Themessage may include an advice to take a medicine or to contact a doctor.

FIG. 10 is a flow chart diagram showing an example method 1000 formonitoring respiratory rate of a patient. The method 1000 may beimplemented in system 100. In block 1010, the method 1000 may commencewith providing, by a gyroscope of a wearable device disposed on a wristof a patient, a gyroscope signal indicative of a motion of the patient.The gyroscope can be communicatively coupled with a processor. In block1020, the method 1000 may proceed with performing, by the processor, aspectral analysis of the gyroscope signal to obtain a spectrum in apre-determined frequency range, the pre-determined frequency rangecovering a normal respiratory rate range. In block 1030, the method 1000may determine, by the processor, a position of a peak in the spectrum toobtain a value for a respiratory rate. In block 1040, the method 1000may provide, by the processor and based on the value of the respiratoryrate, a message regarding the health status of the patient. In someembodiments, the method 1000 may determine, by the processor, that thevalue of the respiratory rate is outside of the normal respiratory raterange and provide, by the processor, an alert message regarding thehealth status of the patient based on the determination that the valueof the respiratory rate is outside of the normal respiratory rate range.The alert message may include a warning against upcoming or worsening ofa chronic disease from which the patients suffers, for which respiratoryrate is one of the indicators. Upon receiving the message, preventivemeasures can be taken to reduce the severity of upcoming or worsening ofa chronic disease. The message may include an advice to take amedication or to contact a doctor.

The present technology is described above with reference to exampleembodiments. Therefore, other variations upon the example embodimentsare intended to be covered by the present disclosure.

What is claimed is:
 1. A system for monitoring an oxygen saturation of apatient, the system comprising: a wearable device configured to bedisposed around a wrist of the patient, the wearable device comprising:a first optical sensor configured to measure, at a palmar surface of thewrist, a first red wavelength photoplethysmography (PPG) signal and afirst infrared wavelength PPG signal; and a second optical sensorconfigured to measure, at a dorsal surface of the wrist, a second redwavelength PPG signal and a second infrared wavelength PPG signal; and aprocessor communicatively connected to the first optical sensor and thesecond optical sensor, the processor being configured to: determine thatconditions for calibration of the first optical sensor are satisfied; inresponse to the determination: determine, based on the second redwavelength PPG signal and the second infrared wavelength PPG signal, afirst ratio for obtaining the oxygen saturation; and determine, based onthe first ratio, a first parameter for modifying the first redwavelength PPG signal and a second parameter for modifying the firstinfrared wavelength PPG signal; determine a second ratio for obtainingthe oxygen saturation, the second ratio being determined based on thefirst red wavelength PPG signal, the first infrared wavelength PPGsignal, the first parameter for modifying the first red wavelength PPGsignal, and the second parameter for modifying the first infraredwavelength PPG signal; determine, based on the second ratio, a value ofthe oxygen saturation; and provide, based on the value of the oxygensaturation, a message regarding a health status of the patient.
 2. Thesystem of claim 1, wherein the determining that the conditions forcalibration of the first optical sensor are satisfied includesdetermining that the patient is wearing the wearable device on thewrist.
 3. The system of claim 1, wherein the determining that theconditions for calibration of the first optical sensor are satisfiedincludes determining that the value of the oxygen saturation is outsidea pre-determined range.
 4. The system of claim 1, wherein thedetermining that the conditions for calibration of the first opticalsensor are satisfied includes determining that a first signal-to-noiseratio (SNR) of the second red wavelength PPG signal exceeds apre-determined threshold and a second SNR of the second infraredwavelength PPG exceeds the pre-determined threshold.
 5. The system ofclaim 1, wherein the determining the second ratio includes: shifting thefirst red wavelength PPG signal by the first parameter; shifting thefirst red infrared wavelength PPG signal by the second parameter; andcalculating the second ratio based on the shifted first red wavelengthPPG signal and the first infrared wavelength PPG signal.
 6. The systemof claim 5, wherein the first parameter and the second parameter aredetermined by formula R_(a)(L^(red), L^(ir))=R_(b), wherein the L^(red)is the first parameter, the L^(ir) is the second parameter, the R_(a) isthe second ratio, and the R_(b) is the first ratio.
 7. The system ofclaim 1, wherein the first optical sensor is configured to measure thefirst red wavelength PPG signal and the first infrared wavelength PPGsignal is substantially near a radial artery of the wrist.
 8. The systemof claim 1, wherein the processor is further configured to: use thefirst ratio to correct the second ratio prior to the determination ofthe oxygen saturation; and calculate the oxygen saturation based on thecorrected second ratio.
 9. The system of claim 8, wherein the correctedsecond ratio is found by formula R=α R_(a)+(1−a)R_(b), wherein the α isa weight between 0 and 1, the R_(a) is the first ratio, and the R_(b) isthe second ratio.
 10. The system of claim 1, wherein the processor isfurther configured to: modify, using the first parameter, the first redwavelength PPG signal; modify, using the second parameter, the firstinfrared wavelength PPG signal; and analyze the modified first redwavelength PPG signal and the modified first infrared wavelength PPGsignal to obtain one of the following: a heart rate and a cardiacoutput.
 11. A method for monitoring an oxygen saturation of a patient,the method comprising: measuring, by a first optical sensor of awearable device and at a palmar surface of a wrist of the patient, afirst red wavelength photoplethysmography (PPG) signal and a firstinfrared wavelength PPG signal, the wearable device being configured tobe disposed around the wrist of the patient; measuring, by a secondoptical sensor of the wearable device and at a dorsal surface of thewrist of the patient, a second red wavelength PPG signal and a secondinfrared wavelength PPG signal; and determining, by a processorcommunicatively connected to the first optical sensor and the secondoptical sensor, that conditions for calibration of the first opticalsensor are satisfied; based on the determination: determining, by theprocessor and based on the second red wavelength PPG signal and thesecond infrared wavelength PPG signal, a first ratio for obtaining theoxygen saturation; and determining, by the processor and based on thefirst ratio, a first parameter for modifying the first red wavelengthPPG signal and a second parameter for modifying the first infraredwavelength PPG signal; determining, by the processor, a second ratio forobtaining the oxygen saturation, the second ratio being determined basedon the first red wavelength PPG signal, the first infrared wavelengthPPG signal, the first parameter for modifying the first red wavelengthPPG signal, and the second parameter for modifying the first infraredwavelength PPG signal; determining, by the processor and based on thesecond ratio, a value of the oxygen saturation; and providing, by theprocessor and based on the value of the oxygen saturation, a messageregarding a health status of the patient.
 12. The method of claim 11,wherein the determining that the conditions for calibration of the firstoptical sensor are satisfied includes determining that the patient iswearing the wearable device on the wrist.
 13. The method of claim 11,wherein the determining that the conditions for calibration of the firstoptical sensor are satisfied includes determining that the value of theoxygen saturation is outside a pre-determined range.
 14. The method ofclaim 11, wherein the determining that the conditions for calibration ofthe first optical sensor are satisfied includes determining that a firstsignal-to-noise ratio (SNR) of the second red wavelength PPG signalexceeds a pre-determined threshold and a second SNR of the secondinfrared wavelength PPG exceeds the pre-determined threshold.
 15. Themethod of claim 11, wherein the determining the second ratio includes:shifting the first red wavelength PPG signal by the first parameter;shifting the first red infrared wavelength PPG signal by the secondparameter; and calculating the second ratio based on the shifted firstred wavelength PPG signal and the first infrared wavelength PPG signal.16. The method of claim 15, wherein the first parameter and the secondparameter are determined by formula R_(a)(L^(red), L^(ir))=R_(b),wherein the L^(red) is the first parameter, the L^(ir) is the secondparameter, the R_(a) is the second ratio, and the R_(b) is the firstratio.
 17. The method of claim 11, wherein the first optical sensor isconfigured to measure the first red wavelength PPG signal and the firstinfrared wavelength PPG signal is substantially near a radial artery ofthe wrist.
 18. The method of claim 11, further comprising prior todetermining the oxygen saturation, using, by the processor, the firstratio to correct the second ratio; and calculating, by the processor,the oxygen saturation based on the corrected second ratio.
 19. Themethod of claim 18, wherein the corrected second ratio is found byformula R=α R_(a)+(1−a)R_(b), wherein the α is a weight between 0 and 1,the R_(a) is the first ratio, and the R_(b) is the second ratio.
 20. Anon-transitory computer-readable storage medium having embodied thereoninstructions, which when executed by a processor, perform steps of amethod, the method comprising: acquiring a first red wavelengthphotoplethysmography (PPG) signal and a first infrared wavelength PPGsignal measured at a palmar surface of a wrist of a patient by a firstoptical sensor of a wearable device, the wearable device beingconfigured to be disposed around the wrist of the patient; acquiring asecond red wavelength PPG signal and a second infrared wavelength PPGsignal measured at a dorsal surface of the wrist by a second opticalsensor of the wearable device; and determining, by a processorcommunicatively connected to the first optical sensor and the secondoptical sensor, that conditions for calibration of the first opticalsensor are satisfied; in response to the determination: determining, bythe processor and based on the second red wavelength PPG signal and thesecond infrared wavelength PPG signal, a first ratio for obtaining anoxygen saturation; and determining, by the processor and based on thefirst ratio, a first parameter for modifying the first red wavelengthPPG signal and a second parameter for modifying the first infraredwavelength PPG signal; determining, by the processor, a second ratio forobtaining the oxygen saturation, the second ratio being determined basedon the first red wavelength PPG signal, the first infrared wavelengthPPG signal, the first parameter for modifying the first red wavelengthPPG signal, and the second parameter for modifying the first infraredwavelength PPG signal; determining, by the processor and based on thesecond ratio, a value of the oxygen saturation; and providing, by theprocessor and based on the value of the oxygen saturation, a messageregarding a health status of the patient.